U.S. Pat. No. 4,459,567 provides a dielectric waveguide isolator capable of operating in the millimeter wave frequency range in dielectric waveguide transmission line. This is accomplished by using an hexagonal ferrite as for example, a thin rectangular substrate of barium oxide substituted NiCo ferrite affixed to the side of a dielectric waveguide as the resonance isolator. After having bonded the ferrite to the dielectric waveguide, the unit is placed between the pole pieces of an electromagnet to magnetize and fully orient the ferrite material. After this process is completed, there is no further need of magnetic biasing for the isolator.
The difficulty with the dielectric waveguide ferrite resonance isolator of U.S. Pat. No. 4,459,567 is that heat builds up in the ferrite during the absorption of high millimeter wave power for the reverse direction of energy propagation in the isolator. The ferrite, being a very thin slab of low conductivity material being mounted on a magnesium titanate dielectric waveguide (also having low thermal conductivity) typically heats up and quickly loses its nonreciprocal properties and thus renders the isolator useless under high power operating conditions.